1. Field of Invention
This invention relates generally to projection-receiving surfaces and methods of making same, and more particularly to broadband projection screens that facilitate high contrast in strong ambient light and that supports multiple simultaneous images.
2. Related Art
Projection screens do not perform well in conditions of strong ambient light. The screen-image viewed by observers differs significantly from the original projection image. The measures of poor replication of a projected image, as it is experienced in the associated viewed image, can be any one of, or combination of, objective, quantifiable metric factors. Many of the metrics of quality for an image projected onto a screen are seriously degraded even when ordinary room lighting illuminates a projection screen. The incidence of direct sunlight on a screen typically destroys in totality every metric of the image's quality. Prior art has attempted to overcome the degradation of projection screen metrics, but the complex problem of maintaining high quality under severe environmental lighting conditions has until now eluded solution. Although an art may preserve one or several of the metrics of image quality, each prior art fails, in strong ambient light, to provide preservation of the complete array of metric levels characteristic of high quality imagery.
The challenges for prior art of any genre to realize good levels for all of the image metrics when a screen is used in strong ambient light will be reviewed below. The review will set a framework for understanding the objects, uniqueness, and non-obviousness of the present invention.
With regard to prior art, it is important to bear in mind throughout this review that, although prior art screens have been successful in attaining suitable levels in one or more of the performance metrics, no single prior art screen has been successful in attaining all the metrics at the same time when exposed to strong ambient light. In this review, only a limited number of examples can be included from applicable prior art genre to illustrate each genre's ability to attain needed levels in each metric. The examples are not intended to be exhaustive in specifics. Rather, they are intended to be inclusively representative of the genre to which they correspond and to provide a vehicle for understanding the intricacies of the metrics individually and interactively.
The array of metrics includes more than mere consideration of screen image brightness. Indeed, once image brightness exceeds visual threshold, preferably obtained using an efficient reflector screen, the quality of a projected image as it appears on the screen is governed by prevention of glare and speckle, by maintenance of contrast of the image being projected, by conservation of gray scale linearity, by maintenance of image brightness uniformity, by fidelity in reproduction of color hue and color saturation, and by preservation of the original projection image's resolution. Additionally, the avoidance of Moire patterns, the minimization of depolarization, and achievement of broadband performance have significant value in a high performance screen.
Genre of Prior Art. The aforementioned requirements for obtaining quality projected imagery in high ambient light will be reviewed with respect to prior art by roughly dividing prior art into three genre: Traditional unitary-gain diffuse scattering screens, Diffraction-based screens, and Enhanced-gain screens with either catadioptric architectures (combines reflection and refraction) or completely reflective morphologies.
Traditional unitary-gain projection screens produce a viewing volume (region wherein observers satisfactorily see the image light on the screen) that includes the entire hemisphere in front of the screen. This is achieved by way of light diffusion, light scatter, or a combination of both. The finish (surface roughness) of the screen, not the figure (shape) of the screen governs the reflection's angular profile. A significant and relatively constant percentage of the reflected image is seen in every direction. The theoretical screen with such a brightness profile is the Lambertian screen, which scatters the collective of the image light isotropically into the front hemisphere. Paint, powders, papers, plastics and crinkled metal are typical screen surfaces that loosely approximate the Lambertian performance. As will be understood from discussion below, unitary-gain screens are not amenable to operation in strong ambient light.
Diffraction-based screens use the phenomenon of light wave interference to produce the screen itself, or to cause the preferred constructive reflection of the light into a viewing volume, with or without the gain described below. For many reasons diffraction-based screens are poor candidates for operation in strong ambient light.
Enhanced-gain projection screens deliver a greater brightness to some viewer locations than unitary-gain screens deliver to the same locations. This is the result of redirecting light from one part of the Lambertian volume so that it adds to another part. In effect, a notable portion of the projection light is redirected (dispersed) under the influence of the figure (shape) of projection screen surface components. Catadioptric screen systems achieve enhanced gain, shown as 8 in FIG. 1, using a refractive process and a reflective process. First, incoming projection light impinges on a refractive layer (lens array, optical beads, prisms, etc.) that deviates the path of the light. The light then continues on to a second layer. That layer reflects the light back out through the refractive layer, from which the light exits into a smaller viewing volume than with a Lambertian screen. (Other catadioptric screen systems have varying details, but operate on a similar concept.) Later discussion will disclose the shortcomings of catadioptric screens for obtaining quality mages in strong ambient light. Much of the problem can result from residual diffusion areas, though inability to obtain desired reflection-element figure for controlled dispersion can be a factor also.
Enhanced-gain projection screens using purely reflective morphologies have a front-surface array of small dispersion cells on the screen that reflects the projection light into a reduced viewing volume by way of an array of mirrorlettes having pre-selected curvatures or slopes on the continuous reflective surface itself, or by orientation of individual reflectors dispersed within a medium at the screen where the purpose of the medium is to hold the reflectors, not to refractively redirect the light as with catadioptric screen systems. Prior art using reflective morphologies have offered the best route for operation in strong ambient light. No reflective-morphology prior art has preserved high enough performance in all the metrics needed for maintenance of quality imagery under such conditions, however. This is so even though the diffuse scatter has been reduced to a fraction of the dispersed (redirected) light; it just cannot be reduced enough with prior art to solve the problem of strong ambient light.
The foregoing review of the three genre for prior art is by no means exhaustive of details, nor are the demarcation parameters used for placement of specific prior art examples into each genre definitive or absolute. Still, the foregoing review provides a fair and complete representation of the concepts used throughout the realm of prior art.
From this review, prior art's shortcomings for creation of high-contrast images in strong-light environments can be discussed. This discussion now will be undertaken relative to the earlier mentioned metrics, namely: efficiency in production of bright images, prevention of glare and speckle, maintenance of contrast of the image being projected, conservation of gray scale linearity, maintenance of image brightness uniformity, fidelity in reproduction of color hue and color saturation, and preservation of the original image's resolution. To these will be added discussion of the previously mentioned need for avoidance of Moire patterns, minimization of depolarization, achievement of broadband performance, and realization of commercial viability.
Below is a discussion of the prior art vis-à-vis individual metrics.
Regarding efficient production of screen brightness using prior art: In any case wherein the viewers do not need to be distributed throughout the entire forward hemisphere of the screen, a unitary gain screen as illustrated in FIG. 1 curve 11, is not light efficient. An enhanced gain screen is more efficient, as shown by FIG. 1 curves 12 (dotted line curve), 14 (solid line curve) and 16 (dashed curve). It reflects more light power to viewer locations and less to the peripheral angles where viewers are confronted by unacceptable image distortion even if they did receive reflected light. Herein, gain 8 is defined as any increase above curve 11, as shown in FIG. 1. Gain generally is the ratio of screen brightness to Lambertian brightness for equivalent levels of projection light. Lens arrays such as U.S. Pat. No. 4,606,609 to Hong (1986), U.S. Pat. No. 4,767,186 to Bradley, Jr., et al. (1988), and U.S. Pat. No. 4,911,529 to Van De Ven (1990), along with various glass bead architectures, all as representative of catadioptric screen systems, as well as silver reticulated surfaces as representative of an enhanced gain reflective screen, are typified by FIG. 1 curves 12 and 14. Glass bead screens are the more common in the marketplace. They have various forms of backing material composition and morphology (shaping). Gains achievable with glass bead screens are restricted by intrinsic limits of spheres in allowing active area packing density. Gains with metric values approaching three can be achieved, although such gains are uncommon. Other catadioptric screens can demonstrate higher gains but generally not with the often-desired profile of FIG. 1 curve 16. As with glass bead screens, the other catadioptric screens are not suitable in additional regards, such as broadband spectral operation and maintenance of image darkness as needed to preserve high contrast ratios.
The dispersion of light is actually a three-dimensional issue, so the power efficiency of a screen can be much higher than the two-dimensional portrayals in FIG. 1 would suggest. In a two-dimensional portrayal the area under each dispersion curve is maintained constant to satisfy the principle of conservation of energy. In a three-dimensional portrayal, conservation of energy requires that the volume under each screen's dispersion surface be maintained. This can emphasize the differences in screen gain of FIG. 1. The additional dimension also has a major effect on the performance of a screen when deflecting strong environmental light away from the viewing volume.
Mirrorlettes, in theory, can yield much higher screen gains than catadioptric screens. Performance such as FIG. 1 curve 16 would be theoretically attainable only with mirrorlettes, and only if suitable manufacturing hurdles could be overcome, which have not currently been overcome. However, prior art attempts to make large mirrorlette arrays with the profiles of curve 16 have been unsuccessful because the basic concepts associated with proposed assemblies failed to account for the difficulties of making an array of even hundreds of thousands, let alone the needed millions, of very small, optical quality elements.
The requirements for individual mirrorlettes can be considered on two scales of dimension. One scale is associated with the overall size and curvature of the mirrorlette, which is often termed “figure.” The outer dimensions of an individual mirrorlette figure are many times larger in size than the wavelength of the light it is designed to reflect. That is, the figure is the intended shape, controlled to tolerances that are allowed to be very much larger than a wavelength of light for which the mirrorlette is being designed. The figure of the mirrorlette surface within the outer dimensions of the mirrorlette is designed to produce the screen's desired light dispersion pattern, with the presumption that specular reflection can be achieved by the surface. A metric associated with screen mirrorlette figure is based on this assumption of a surface finish that produces perfect specularity. The fraction of light falling within the obtained mirrorlette shape relative to the desired distribution is the figure quality. It should be noted that this metric is not a measure of smoothness; it is a measure of a mirrorlette's ability to approach desired or theoretical distribution of the projector's light if a specular surface exists.
Specular reflection is to be understood here as that component of reflected light that lies along the line that is symmetric about the reflective surface's normal. Accordingly, specular reflection as used herein is that reflected component that follows the simple textbook relationship where the magnitude of the angle of reflection is equal to the magnitude of the angle of incidence when measured from the normal (perpendicular) to the surface where the incident light impinges; wherein the plane of angle measurement includes the incident light ray, the surface normal, and the reflected light ray. The specular reflection angle is equal in magnitude but opposite in sign to the angle of incidence because they are symmetric about the surface normal. (For practical application, the size of the aperture of a light gathering device influences whether reflected light is deemed a specular component of the overall reflection profile.) According to this geometric definition of specular reflection, even a Lambertian reflector surface has a specular component, albeit that component representing only a small fraction of the total amount of light reflected by the surface.
The ability to achieve the specular reflection desired for a mirrorlette surface is associated with a second and much smaller scale of dimension than that of figure: surface roughness, termed “surface finish.” Surface finish quality is measured relative to the surface roughness as examined on a scale with units near the wavelength of the light to be reflected. A metric for mirrorlette surface finish is rms surface roughness in wavelength units. FIG. 2 suggests the two-dimensional relationship between surface finish and the fraction of light that is not reflected specularly. The curves show the spread of reflection for root-mean-square (rms) finishes of a tenth 18, a half 20, one 22 and two 24 wavelengths relative to the ideal specular line 26. The ability of a projection screen of the mirrorlette type to operate in a bright ambient environment is heavily dependent on both a proper figure and an exceptional specularity in surface finish. The need for the latter is emphasized even further when depicted in three dimensions, rather than just the two dimensions of FIG. 2. For visual light the scale of surface finish must be considerably sub-wavelength to reduce the non-specular reflection to a level that allows enough deflection of strong light away from the viewing volume to maintain high image quality. Typically, the rms surface roughness for visual light must be less than 0.1 micrometer. The rms of 0.1 micrometer for visual light results in curve 18.
Routine machine cutting and grinding techniques used to cut an individual mirrorlette figure yield far rougher surfaces than 24. This is far from the optical quality finish needed to produce extreme levels of specularity. Accordingly, optical polishing must follow machine cutting and machine-grade polishing of mirrorlette elements. Generally this process is via a sequence of smaller and smaller grit abrasives used on each of the mirrorlettes, which necessitates extreme alignments and sequencing for many thousands of small mirrorlettes. Short of optical polishing, chemical and electronic etching techniques might be employed to achieve the desired optical surface. However, for several reasons, including micrometer-sized variations in chemical reactivity for chemical processes and electric field variations for electronic processes, neither of these options is readily applicable to achieving curve 20 or better on a large scale in the laboratory, let alone is any of them commercially practical. Similar phenomena negate the use of plating techniques to achieve an infilling of roughness that remains after machining of the mirrorlettes. (It should be noted that injection molding, etc., replicates most of the machining flaws.) Processes like chrome plating might seem to yield sufficient infilling to yield optical quality surfacing, however, this is far from the case. Pleasing shininess is achievable with curve 24, but it does not support high resolution. Neither does it support off-axis rejection of bright light (such as sunlight) that seriously degrades resolution, contrast, color saturation and polarity. Still, it should be pointed out that extreme surface finish is not a requirement for high screen gain; it is a requirement for bright light rejection.
Prior art for high-gain screens has been based on developing a surface of appropriate figure, then imparting to that figure a surface of proper finish. In some cases, such as injection molding and electroform replication of a master tool, the surface finish is imparted concurrently with the imposition of surface figure. Although the surface figure may be adequate for achieving screen gain, the finish (curve 24 or worse) is not adequate for rejection of bright lights (which requires finish of curve 20 or better). Even a small residual of diffuseness resulting from less than optical quality sub-wavelength finish will negate the ability to reject strong environmental lighting. That is, the key to screen gain with rejection of unwanted backgrounds rests more with making the fraction of light in the diffusely scattered light component smaller than with making the fraction of light in the directively dispersed light component greater. This is a very crucial nuance for screen technology.
Another hurdle associated with machine-made and subsequently optically polished mirrorlettes is the extreme difficulty in fabrication of convex mirrorlettes. The margins between one mirrorlette and its neighbors often must be very thin (fifty micrometers or less for a one-millimeter mirrorlette). Such narrowness in an array of tens or hundreds of thousands of mirrorlettes per square meter is exceedingly difficult to maintain. With concave mirrorlettes, a curved cutting tool easily and consequently produces narrow edges, but these are ridges that are easily damaged in situations associated with common use of projection screens. (Further, such ridges have electrostatic consequences.)
Another shortcoming of prior art is its inability to provide a high-quality screen with real-time variable gain. While prior art facilitates some forms or real-time change to the gross shape of an entire screen, as with U.S. Pat. No. 4,022,522 to Rain (1977); this is of no value to modification of gain. In essence, prior art adjustability fails to provide a projection high-gain screen with real-time variable gain.
Regarding prevention of glare and speckle using prior art: Glare is a collective effect whereby large sections of the screen present the desired projected image, but superimposed on the image are large bright patches. Glare patches are similar in general appearance to the glare obtained when the screen patch is simply a standard specular mirror. The apparent physical size of the glare encompasses many of the cells used in the high-gain architecture. This idea is portrayed in FIGS. 1 and 3. The appearance of glare as depicted in 28 (FIG. 1) and 30 (FIG. 3) might be conceptualized in terms of small nearly-planar mirrors set next to each of most of the cells, with the angle on the mirror such that it directly reflects back into the viewer's eyes the nearly collimated light of the projector. A metric for glare is based on the fractional size of a screen region whose periphery 32 is defined by a brightness angular rate-of-change exceeding the screen-design rate of change by at least a factor of two and maintains that excessive brightness, or more, throughout the region, which need not be symmetric. Ideally, a screen will have no glare. The metric assumes projection of a uniformly bright image.
Glare is common to high-gain screen designs that use continuous surface mirrorlettes, such as used in U.S. Pat. No. 4,297,001 to Antes, et al. (1981). Techniques of this nature produce curved specular surface cells with a desirable shape for high gain over the center of each mirrorlette cell, but that also produce notably excessive gain (in essence a glare) rather than uniform dispersion of the projector light. The glare comes from the transition surface between the mirrorlette cells, where the curvature angles are considerably flattened. The cusp-like grooving technique used in the electroforming approach of U.S. Pat. No. 3,994,562 to Holzel (1976) somewhat alleviates glare, but at the severe expense of excessive scattering of off-axis background light due to the finish limits of electroforming. Also, this prior art has practical limitations in seamless screen size. Glare can be reduced using other prior art, such as was employed in U.S. Pat. No. 4,235,513 to Vlahos (1980), wherein the advantage of continuous surface is foregone, making it necessary to individually fabricate and mount each mirrorlette onto a tile and replicate by electroforming with its finish quality limits. A related approach was taken in U.S. Pat. No. 4,040,717 to Cinque, et al. (1977), wherein concave cells were individually constructed, with the attendant disadvantage in production simplicity and the exposure to damage of the cell edges because of the concave architecture. Catadioptric screens, typified in U.S. Pat. No. 4,068,922 to Dotsko (1978), U.S. Pat. No. 4,298,246 to Iwamura (1981), U.S. Pat. No. 4,606,609 to Hong (1986), and U.S. Pat. No. 5,625,489 to Glenn (1997), along with others, generally still display glare from the binding materials or lenslette front-surface effects, but at least the glare tends to be at more acceptable levels than with other prior art. However such screens perform poorly in many of the other screen metrics when they are operated in strong ambient light.
As mentioned previously, glare 28 and 30 is a collective property of the screen element area and speckle 34 and 36 is a localized area effect, as depicted in FIGS. 1 and 2. Speckle is the result of a notably higher gain at a small location separated in the observer's view by several visual resolution elements. A metric for screen speckle is, assuming uniform projected image brightness, the number of speckled resolution elements relative to the total number of resolution elements. A good design goal is for the speckle count to remain below one in ten thousand.
Speckle appears like a star-field superposition of bright spots on the image. As in the case of stars, speckle points as focused on the retina need not be as large as a retinal cell; they simply need to evoke a response in the cell that is notably larger than the evocation from light on neighboring clusters of cells. A common tactic in prior art reduces speckle by making screens with a multitude of scattering elements in an area very much smaller than the eye's resolution. Thus, the speckle is reduced by integration of many random scatterers. However, as expected with stochastic events, probabilities exist that an occasional integration region will still be excessively brighter than its neighbors and appear as a speckle. This can be observed in most glass bead screens and many reticulated screens.
Regarding control of angular cutoff rate using prior art: The ability to control the angular cutoff rate 38 (FIG. 1), wherein the notch shows a change in brightness relative to a change in angle, is important to the rejection of strong ambient light. Further, an extreme cutoff rate such as depicted in FIG. 1 curve 16, allows a screen to present different images to different viewing volumes, without interference between the images, by using multiple projectors located at different angular locations as suitable to the viewing volume of each audience. Prior art, such as U.S. Pat. No. 5,112,121 to Chang, et al. (1992), attempt to achieve rapid enough cut-off to support distinct simultaneous viewing volumes via multiple offset projectors. In the case of Chang, a holographic screen is used. By the very nature of a holographic screen, it is exceedingly vulnerable to ambient light, will not sustain color integrity, has significant brightness sidelobes, and is highly sensitive to mechanical displacement.
A metric for angular cut-off rate is relative brightness change per degree of angle offset away from the optical axis of the specularly reflected projection beam. The ability to achieve cut-off rates of 99% per degree at the edge of the viewing volume is a desirable screen feature for operation in strong ambient light. Unitary-gain screens have no facility in this regard. Enhanced-gain screens of the catadioptric type using prior art have no significant capability for attaining sharp cutoff and wide viewing angles simultaneously. Enhanced-gain reflective mirrorlette prior art architectures allow somewhat more angular viewing volume control. However, their rate of cutoff, when scaled as a fraction of the total angular extent, is not selectable; nor does it approach 99% per degree on all viewing volume edges. Prior art does not accommodate decoupling of the ability to control angular cutoff rates from the ability to control angular viewing volume.
Regarding maintenance of contrast darkness using prior art: A metric for overall image contrast is the ratio of the brightness measured in the lightest area of the image to the brightness measured in the darkest part of the image. In order to efficiently provide a viewer with a high-contrast projected image in bright ambient light, the availability of high-screen gain is not sufficient. This is because contrast is not driven by brightness alone. Contrast is also the result of how well the screen can reproduce the dark elements of the image. FIG. 4 assists understanding of this fact. If the brightest area 40 of the projected image has an inherent intensity of 10 units and the darkest area 42 an inherent intensity of 1 unit, then the overall brightness contrast ratio is 10, as depicted in FIG. 4 curve 44.
Table Associated with Curve 44 of FIG. 4Contrast for Picture Element Having Listed Projected Light LevelAssume Minimum Projected Light to Be 1 UnitBackground (non-image) Light Seen by Observer is Constant at Zero UnitsBackgroundProjected Image Light(Non-image Light)(Arbitrary Units)(Arbitrary Units)Contrast EquationContrast10(1 + 0)/(1 + 0)1.0020(2 + 0)/(1 + 0)2.0030(3 + 0)/(1 + 0)3.0040(4 + 0)/(1 + 0)4.0050(5 + 0)/(1 + 0)5.0060(6 + 0)/(1 + 0)6.0070(7 + 0)/(1 + 0)7.0080(8 + 0)/(1 + 0)8.0090(9 + 0)/(1 + 0)9.00100(10 + 0)/(1 + 0) 10.00
Table Associated with Curve 48 of FIG. 4Contrast for Picture Element Having Listed Projected Light LevelAssume Minimum Projected Light To Be 1 UnitBackground (Non-image) Light Seen by Observer is Constant at 10 UnitsProjected ImageBackgroundLight(Non-image) Light(Arbitrary Units)(Arbitrary Units)Contrast EquationContrast110(1 + 10)/(1 + 10)1.00210(2 + 10)/(1 + 10)1.09310(3 + 10)/(1 + 10)1.18410(4 + 10)/(1 + 10)1.27510(5 + 10)/(1 + 10)1.36610(6 + 10)/(1 + 10)1.45710(7 + 10)/(1 + 10)1.55810(8 + 10)/(1 + 10)1.64910(9 + 10)/(1 + 10)1.731010(10 + 10)/(1 + 10) 1.82
If unwanted environmental light scatters as little as 5 units of light into the viewer's vision, as shown in curve 46, then the brightness contrast ratio deteriorates to (10+5)/(1+5)=2.5. For a brighter environmental source, such as modest skylight, 10 units of intensity might scatter into the viewer space, for which the mirrorlettes are designed, as shown in curve 48. This yields degradation from the original 10:1 brightness contrast ratio down to 1.8:1. For impingement of direct sunlight on a high-gain glass bead screen the ratio typically deteriorates to 1, which means the complete loss of a viewer's ability to see the projected image. Clearly, the dark components in a projected image are the most vulnerable to strong environmental light.
The majority of prior art screens have been relegated to use in subdued lighting, or to accepting the multiple metric deficiencies using the limited capabilities of prior art attempts to design screens suitable to strong ambient light. Prior art such as U.S. Pat. No. 6,384,970 to Abe, et al. (2002), U.S. Pat. No. 4,235,513 to Vlahos (1980), U.S. Pat. No. 4,298,246 to Iwamura (1981), U.S. Pat. No. 4,767,186 to Bradley, Jr., et al. (1988), U.S. Pat. No. 4,911,529 to Van De Ven (1990), U.S. Pat. No. 6,040,941 to Miwa, et al. (2000), to indicate just a few, have fitted screen elements with light-absorbing baffles or light-absorbing masks, with modest increases in the level of environmental lighting that can be tolerated without undue deterioration of the projected image's contrast. Taking a different tack on the problem, U.S. Pat. No. 5,210,641 to Lewis (1993) goes so far as to produce an angular pass filter to overlay optical cells for absorbing or rejecting off-axis light. Other prior art, such as U.S. Pat. No. 5,296,965 to Uetsuki, et al. (1994), U.S. Pat. No. 5,335,022 to Braun, et al. (1994) and U.S. Pat. No. 5,625,489 to Glenn (1997) try to subdue ambient light by using mismatched polarization with screens relative to room lights. These latter prior arts unfortunately destroy the ability to employ polarization advantages that are discussed later, and they also decrease screen gain.
Deflection of impinging bright non-projector light away from the viewing volume requires an optical surface and a cell figure that is not practically achievable for large arrays of small mirrorlettes using prior art. While prior art finishes may be acceptable for modest environmental lighting levels, they are deficient for bright sources, such as direct illumination with car lights, daylight, and sunlight.
The number of patents in this arena is a clear indicator of the importance placed on background rejection. However, prior art for background rejection including the use of mirrorlettes fails to achieve suitable metric levels for the complete set of other desirable screen attributes.
Regarding conservation of gray scale linearity (and quantization) using prior art: In the above discussion of contrast maintenance under bright ambient lighting conditions only the effects on the extremes of darkest and lightest image areas were considered. However, the same effect is imposed on all the intermediate levels of brightness within an image. The result is a delinearization of the original image's gray scale. That is, the ratios of brightness for various areas in the image as seen by a viewer are changed from the ratios of brightness provided by the projector for those very same areas. This loss of projected image grayscale fidelity demonstrates that the effect of the ambient light on contrast ratios is not limited only to the areas of maximum image brightness and darkness. The loss of gray scale linearity is depicted in FIG. 4 curves 46 and 48. On curve 44 (which does not have gray scale linearity loss) can be seen levels of viewed image brightness corresponding to levels 2, 4, 6, 8 and 10 units of projected image brightness. For a darkened environment, the ratios for viewed image brightness will match those of the projection itself. However, for ambient lighting conditions 46 and 48, the ratios associated with viewed image brightness do not match those for the projection image brightness. The linearity of the image gray scale has been lost. Prior art offers help in this regard to some extent by increasing the screen gain. This allows the fractional impact of the ambient light to be reduced. However, for very bright environments the ability of prior art to conserve gray scale linearity is significantly limited for the same reasons presented in the contrast discussion. Indeed, for direct sunlight conditions the projection image using prior art would need to be so bright as to keep a viewer from even looking at it. Further, the needed projection lamp power would likely burn up the image storage medium.
A metric for conservation of gray-scale linearity is the ratio of image contrast slopes. These are obtained by calculating the contrast range of the viewed image divided by the contrast range of the projected image. The grayscale linearity for curves 44, 46 and 48 in FIG. 4 are 1.0, 0.25 and 0.18, respectively. Values below 0.5 noticeably degrade viewed image quality relative to the projected image.
Regarding maintenance of image uniformity using prior art: In general, projection screens are deemed better if they support a uniformity of brightness across their extent. If the screen changes its image brightness slowly with angle, then the uniformity is often acceptable to a viewer. If it falls off too rapidly, then the viewer may not be pleased with the image. This fall-off is different than that associated with purposeful use of screen gain, and is not the same as glare effects and speckle. However, one metric for image uniformity is a variation on the aforementioned image glare metric. For uniformity, the glare metric is applied with various brightness differentials, as suits the aims of the screen designer. A screen art should be capable of producing various uniformity metric values. This is not the case with the majority of prior art.
FIG. 3 shows intensity isocontours 51 illustrating uniformity 50, and image non-uniformity in the form of glare 30, and speckle 36 as they are associated with the rate of change in image brightness produced by the screen. Prior art addresses uniformity roll-off in many ways, sometimes at the expense of other screen metrics. For example, in U.S. Pat. No. 5,541,769 to Ansley, et al. (1996) the provision of uniform brightness is so important as to sacrifice brightness by purposely applying absorption to diminish the amount of light reflected from areas that are brighter than others. This means that the maximum brightness level is governed by the poorest unadjusted element, which provides for very inefficient use of projection energy. With mirrorlette screens, prior art imparts curvature to the overall screen to maintain brightness uniformity across the screen as seen within the viewing volume.
By definition, unitary-gain Lambertian screens will be uniform, but projected image quality will suffer greatly from scattering of ambient light into the viewing volume. The ambient light might not impinge the screen with the same brightness everywhere on its surface. In such circumstances, the result will be a non-uniform viewer image. The same is true for all diffusion screens that approximate Lambertian light-scattering profiles.
Regarding reproduction of color saturation and color hue using prior art: When a bright ambient light is scattered into the viewing volume, the ambient light that adds to the image-forming projection light can change the saturation and hue of the image color. If the impinging ambient light is white, then its addition causes the image color to lose its saturation and become more pastel. If the ambient light is not very white, and therefore relatively saturated, then any difference in color from the image light causes a shift of the image hue toward an intermediate hue. These effects are illustrated in FIG. 5, wherein the hue 52 of the color is indicated by the angle to the color as measured between a radial reference line 54 from the center of the circle, and the saturation 56 is highest at the radial distance of the circumference. The length L1 of saturation line 56 defines the magnitude of saturation. Neutral color from black through white is represented by the circle's center point 57. FIG. 5 depicts the desaturation and the hue shift of an image color 58 as separate events caused by ambient light color 60 scattering into the system.
However, ambient light generally has characteristics that cause both desaturation and change in hue of projected images at the same time. The amount of radial displacement 62 is a metric for saturation fidelity, with a smaller amount of radial displacement 62 being more desirable. The length L2, of radial displacement line 62 defines the magnitude of saturation fidelity. The amount of angular displacement is a metric for hue fidelity, with a smaller amount of angular displacement being more desirable, and having an angular length L3 defining the magnitude. Ambient light color 60 mixes with color 58, causing angular displacement L3, resulting in color 64.
Prior art can enhance screen gain and thereby increase the amount of light from the projected image relative to ambient sources. This reduces, but does not negate, the detrimental effects of the ambient light. And prior art is unable to drastically reduce color desaturation and hue shift while still retaining the other needed attributes that are being delineated in this discussion of image metrics. In fact, some techniques used to increase screen gain, as with glass bead and lens array catadioptric screens, introduce additional problems, such as chromatic effects due to the beads having different refractive indices for different colors. Reflective mirrorlettes using prior art can be more effective in reducing desaturation and hue shift, but not at the same time that glare and speckle are defeated.
Regarding preservation of resolution using prior art: Another effect of ambient light is loss of image resolution, which manifests itself to a viewer in many ways, including added difficulty in perceiving faint objects near bright objects and reducing the ability to separate fine detail. One metric useful for resolution issues is to determine the smallest size of high-contrast parallel lines (equal in width and spacing) in the projected image that will be sustained with an acceptable contrast level in the viewed image. Because of the scattering of light transversely within many unitary gain screen designs, such as plastic diffusion films, there is considerable loss in ability to see small detail. The brighter spots of light diffuse into nearby darker spots of the projected imagery. This effect, also noted in catadioptric screens, deteriorates the delineation of object edges and masks over small image elements altogether. Also, it is to be remembered that resolution by human vision is a function of brightness differentials between neighboring image elements and maintenance of original image brightness profiles, such as grayscale linearity. Thus, while prior art mirrorlette screens can offer reduction in transverse scatter compared to unitary-gain screens and catadioptric screens, the glare components of prior art mirrorlette screens will deteriorate image resolution as well. Further, mirrorlette screens based on drawing-out melted plastic shapes are vulnerable to surface striations and inhomogeneities that add to glare and speckle.
Some prior art mirrorlette concepts require polishing of individual mirrors that then serve as the tools for replication using technologies such as electroforming. Because of the practical size considerations for such optical polishing processes, the resultant mirrorlette cell sizes are too large for close viewing as would be characteristic of conference rooms, in-door motion picture theaters, home entertainment centers, and simulators. This limitation leads to an inability to maintain the resolution of projected images, and in cases of the larger mirrorlette sizes, will lead to Moire patterns.
Regarding avoidance of Moire patterns using prior art: Some inventors of prior art, such as Antes, U.S. Pat. No. 4,297,001 to Antes, et al. (1981), have suggested the need for randomness of centers and sizes for dispersion elements in mirrorlette screens. Accordingly, randomness is a feature in the Antes invention. Modern sampling theory shows this suggestion to be without merit, however. In fact, it is image-sampling frequency that matters, whether or not the light dispersion centers are randomly placed. In fact, a good metric for Moire prevention is the factor by which the spatial frequency of placement of the screen dispersion centers exceeds an information-theory criterion called the Nyquist sampling rate. Prior art in mirrorlette techniques cannot provide a high metric value in this arena, for typical requirements of screen resolution and viewing distance, and still demonstrate good values in the other metrics. Also, with a fabrication process such as delineated in Antes, the randomness, if it could be obtained, would work against rapid angular cut-off, against avoidance of speckle, and against uniformity of brightness because in a truly random case the radius of curvature of the bubbles upon which the invention relies would be different for every mirrorlette. The radius of curvature of the mirrorlette, and the angle the optical axis of the projector makes with the normal to the mirrorlette surface at the mirrorlette edges, are what govern the dispersion angle for mirrorlette techniques.
Regarding minimization of depolarization using prior art: Beyond the aforementioned shortcomings of catadioptric screens for bright light environments, it is found that reflections from glass bead screens, and from most lens array screens, do not maintain the polarity of incident light. This is a serious drawback whenever the use of polarized differentiation is desired. For example, the preferred mode for 3D movies is to cast two images simultaneously on a screen. One image is intended for processing by the viewer's left eye and the other for processing by the viewer's right eye. The two images on the screen, each image cross-polarized to the other, are separated out by providing the viewer with glasses having a properly oriented polarizer in each of the two lens openings. The images are appropriately observed in the correct eyes in accord to the cross polarization of the lenses, each of which blocks out the inappropriate of the two images.
However, because a glass bead screen and other catadioptric screens do not adequately maintain the projected image polarizations upon reflection of the projected light, 3D is lost and the gain afforded by the screen for normal viewing is useless. A metric for polarization maintenance is the ratio of the brightness of the viewed image when viewed through a polarizer having alignment with a projected polarized image, to that when the viewer looks through the polarizer rotated 90 degrees (cross-polarized). Prior art screens that demonstrate significant gain and any notable amount of ambient light rejection have polarization brightness ratios of five and less.
The film industry, using prior screen art, had to resort to essentially no-gain screens. This was a major element in the lack of 3D development. The projector intensity had to be so great to overcome the loss in transmission through the viewing glasses that film was overheated and prematurely deteriorated. Also, because even no-gain screens do not maintain fidelity of polarization, the image separation was still poor. The result was overlapping and cross-feeding between the eyes, which gave headaches and eyestrain to the viewers, as well as presented poor imagery.
Silver/Silver lenticular screens provide a gain modestly higher than unity, but shift colors toward the blue and have a tendency toward glare. Yet this screen is still considered by many as the best prior art medium for 3D projection. Mirrorlette screens, in the theory of some prior art, would seem to provide an even greater improvement. However, prior art screens would not provide minimization of depolarization at the same time that glare, speckle, uniformity, resolution, and darkness metrics are met for strong environmental light applications.
Regarding broadband spectral performance using prior art: projection screen utility is not limited to the visual realm of the electromagnetic spectrum. Many simulation systems require a screen that functions in other spectra, such as ultraviolet, near-infrared, and thermal infrared. None of the projection screen prior art that is based on catadioptric techniques can accommodate this range of projection spectra. Enhanced gain in the visual wavelengths is no indicator of similar gain in the other spectral regions. The spectral range cannot be greater the spectrum transmitted by the glass, polymer, or other material used for the refractive elements. Accordingly, a screen such as a glass bead screen not only lacks gain at wavelengths outside of the visual spectrum, it is not even functional outside that spectrum. This means the screen cannot be used in arcades where the designating light from the guns is too far into the ultraviolet or the infrared realms. It also means that glass-bead screens cannot be used for simulation of thermal infrared screens, as in desirable for such activities as military training and night-vision equipment development.
Enhanced-gain mirrorlette screens defined in prior art can increase the range of spectral performance beyond the visual realm, but not with preservation of good metrics for resolution, glare, speckle, uniformity, and sharp angular cutoff.
Regarding commercial viability using prior art: Commercial viability for a projection screen, assuming the existence of a market, includes factors associated with manufacturability, reliability, maintainability, safety, weight, pliability, cost, and other factors as demanded by the aforementioned market.
Unitary-gain screens are readily found in the marketplace, or made from readily available materials such as paint and cloth. Although there may be many variations in unitary screen approaches, few are outlandishly expensive or operationally impractical; and few are likely to fail because of an error in a basic concept. This can be understood by the representatively different approaches illustrated in such U.S. patents as U.S. Pat. No. 4,006,965 to Takada, et al. (1977), U.S. Pat. No. 4,190,320 to Ferro (1980), and the like.
The situation is similar for simple catadioptric and reticulated screens that claim attainment of enhanced gain using uncomplicated fabrication techniques, such as U.S. Pat. No. 4,025,160 to Martinez (1977), U.S. Pat. No. 4,068,922 to Dotsko (1978), U.S. Pat. No. 4,089,587 to Schudel (1978), U.S. Pat. No. 4,191,451 to Hodges (1980), and U.S. Pat. No. 4,206,969 to Cobb, et al. (1980).
Enhanced-gain reflection-only screens have not been as successful in the marketplace as the aforementioned architectures. Production cost is a major factor. For example, an Internet and literature survey to locate a screen having the architecture of U.S. Pat. No. 4,235,513 to Vlahos (1980) was unsuccessful. Further inquiry with three of the largest screen companies in the world (DaLite, Draper and Bedford) also failed to locate a screen of the aforementioned patent's type. Likewise, efforts to find screens based on the concepts of U.S. Pat. No. 4,235,513 to Vlahos (1980) were also unsuccessful. In this latter case, the lack of large-scale commercialization may have had basic technical origins.
Many inventions that hope for reasonable production viability rely on misconceptions and mistaken assumptions. For example, U.S. Pat. No. 4,235,513 to Vlahos (1980) relies on a constant contact angle between bubbles and the ability to stretch a contiguous array of bubbles made of plastic materials. The constant angle assumed for the invention only occurs in the plane that includes the centers of curvature for two bubbles and that is perpendicular to the tangent of the two bubbles where the plane passes. This angle does not hold for a contiguous array of bubbles and is different as the plane rotates around an axis normal to the plane of the bubble surfaces. Further, the formation of the bubbles and the array is not likely to behave like a group of soap bubbles. Soap bubbles act as they do because they are thin films and surface tension forces dominate over cohesion and gravitational forces. Weights per unit area of surface will be different with molten plastics than with soap bubbles. Clearly, results of experiments with soap bubbles cannot be automatically extended to other bubbles, including the effects of lateral stretching of a contiguous sheet of bubbles.
The purpose of the foregoing discussion of specific patents is not to attack the patents, but rather to illustrate how concepts that look like they are difficult to execute in a commercial sense likely will show themselves ultimately to be difficult to execute in practice. Further, patents based on erroneous assumptions may fail in the marketplace because they do not work as expected.
In the several aforementioned cases one can recognize that mirrorlette arrays have considerable advantages over other approaches for high-gain screens operable in strong environmental lighting. However, the ability to realize a technically and commercially viable mirrorlette array has not been available using prior art. The problem of progress in screen technology has not been a failure to recognize the potential for mirrorlettes. Rather, the problem of progress in screens has been that a viable method of manufacture for such arrays has been elusive, and even an optimal mirrorlette figure has been overlooked for lack of understanding. New inventions of method, of tooling, and of shape were needed to sufficiently address all the aforementioned projection screen metrics. Such are the elements of this present invention.